Transcript Chapter 5 - Department of Computer Engineering

Chapter 5
Link Layer and LANs
These power point slides have
been adapted from slides
prepared by book authors.
Sharif University of Technology
Computer Networking: A Top
Down Approach Featuring the
Internet,
3rd edition.
Jim Kurose, Keith Ross
Addison-Wesley, July 2004.
1
Chapter 5: The Data Link Layer
Our goals:

understand principles behind data link layer
services:





error detection, correction
sharing a broadcast channel: multiple access
link layer addressing
reliable data transfer, flow control: done!
instantiation and implementation of various
link layer technologies
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2
Link Layer





5.1 Introduction and
services
5.2 Error detection and
correction
5.3Multiple access
protocols
5.4 Link-Layer
Addressing
5.5 Ethernet



5.6 Hubs and switches
5.7 PPP
5.8 Link Virtualization:
ATM and MPLS
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3
Link Layer: Introduction
“link”
Some terminology:


hosts and routers are nodes
communication channels that
connect adjacent nodes along
communication path are links




wired links
wireless links
LANs
layer-2 packet is a frame,
encapsulates datagram
data-link layer has responsibility of
transferring datagram from one node
to adjacent node over a link
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Link layer: context

Datagram transferred by
different link protocols
over different links:


e.g., Ethernet on first link,
frame relay on
intermediate links, 802.11
on last link
Each link protocol
provides different
services

e.g., may or may not
provide rdt over link
transportation analogy

trip from Princeton to Lausanne
 limo: Princeton to JFK
 plane: JFK to Geneva
 train: Geneva to Lausanne

tourist = datagram
transport segment =
communication link
transportation mode = link
layer protocol
travel agent = routing
algorithm



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5
Link Layer Services

Framing, link access:



encapsulate datagram into frame, adding header, trailer
channel access if shared medium
“MAC” addresses used in frame headers to identify
source, dest


different from IP address!
Reliable delivery between adjacent nodes



we learned how to do this already (chapter 3)!
seldom used on low bit error link (fiber, some twisted
pair)
wireless links: high error rates

Q: why both link-level and end-end reliability?
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Link Layer Services (more)

Flow Control:


pacing between adjacent sending and receiving nodes
Error Detection:


errors caused by signal attenuation, noise.
receiver detects presence of errors:


Error Correction:


signals sender for retransmission or drops frame
receiver identifies and corrects bit error(s) without resorting
to retransmission
Half-duplex and full-duplex

with half duplex, nodes at both ends of link can transmit,
but not at same time
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Adaptors Communicating
datagram
sending
node
frame
frame
adapter


adapter
link layer implemented in
“adaptor” (aka NIC)


Ethernet card, PCMCI card,
802.11 card

encapsulates datagram in a
frame
adds error checking bits, rdt,
flow control, etc.
receiving side


sending side:

rcving
node
link layer protocol


looks for errors, rdt, flow
control, etc
extracts datagram, passes
to rcving node
adapter is semi-autonomous
link & physical layers
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Link Layer





5.1 Introduction and
services
5.2 Error detection and
correction
5.3Multiple access
protocols
5.4 Link-Layer
Addressing
5.5 Ethernet



5.6 Hubs and switches
5.7 PPP
5.8 Link Virtualization:
ATM
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Error Detection
EDC= Error Detection and Correction bits (redundancy)
D = Data protected by error checking, may include header fields
• Error detection not 100% reliable!
• protocol may miss some errors, but rarely
• larger EDC field yields better detection and correction
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Parity Checking
Single Bit Parity:
Detect single bit errors
Two Dimensional Bit Parity:
Detect and correct single bit errors
0
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0
11
Internet checksum
Goal: detect “errors” (e.g., flipped bits) in transmitted
segment (note: used at transport layer only)
Receiver:
Sender:



treat segment contents as
sequence of 16-bit
integers
checksum: addition (1’s
complement sum) of
segment contents
sender puts checksum
value into UDP checksum
field


compute checksum of received
segment
check if computed checksum
equals checksum field value:
 NO - error detected
 YES - no error detected. But
maybe errors nonetheless?
More later ….
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Checksumming: Cyclic Redundancy Check




view data bits, D, as a binary number
choose r+1 bit pattern (generator), G
goal: choose r CRC bits, R, such that

<D,R> exactly divisible by G (modulo 2)
 receiver knows G, divides <D,R> by G. If non-zero
remainder: error detected!
 can detect all burst errors less than r+1 bits
widely used in practice (ATM, HDCL)
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CRC Example
Want:
D.2r XOR R = nG
equivalently:
D.2r = nG XOR R
equivalently:
if we divide D.2r by
G, want remainder
R
R = remainder[
D.2r
G
]
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Link Layer





5.1 Introduction and
services
5.2 Error detection and
correction
5.3Multiple access
protocols
5.4 Link-Layer
Addressing
5.5 Ethernet



5.6 Hubs and switches
5.7 PPP
5.8 Link Virtualization:
ATM
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Multiple Access Links and Protocols
Two types of “links”:

point-to-point



PPP for dial-up access
point-to-point link between Ethernet switch and host
broadcast (shared wire or medium)



traditional Ethernet
upstream HFC
802.11 wireless LAN
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Multiple Access protocols


single shared broadcast channel
two or more simultaneous transmissions by nodes:
interference

collision if node receives two or more signals at the same
time
multiple access protocol
 distributed algorithm that determines how nodes
share channel, i.e., determine when node can
transmit
 communication about channel sharing must use
channel itself!

no out-of-band channel for coordination
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Ideal Mulitple Access Protocol
Broadcast channel of rate R bps
1. When one node wants to transmit, it can send at
rate R.
2. When M nodes want to transmit, each can send at
average rate R/M
3. Fully decentralized:


no special node to coordinate transmissions
no synchronization of clocks, slots
4. Simple
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MAC Protocols: a taxonomy
Three broad classes:
 Channel Partitioning



Random Access



divide channel into smaller “pieces” (time slots,
frequency, code)
allocate piece to node for exclusive use
channel not divided, allow collisions
“recover” from collisions
“Taking turns”

Nodes take turns, but nodes with more to send can
take longer turns
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Channel Partitioning MAC protocols: TDMA
TDMA: time division multiple access




access to channel in "rounds"
each station gets fixed length slot (length = pkt
trans time) in each round
unused slots go idle
example: 6-station LAN, 1,3,4 have pkt, slots
2,5,6 idle
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Channel Partitioning MAC protocols: FDMA
FDMA: frequency division multiple access





TDM (Time Division Multiplexing): channel divided into N time slots,
one per user; inefficient with low duty cycle users and at light load.
FDM (Frequency Division Multiplexing): frequency subdivided.
frequency bands

channel spectrum divided into frequency bands
each station assigned fixed frequency band
unused transmission time in frequency bands go idle
example: 6-station LAN, 1,3,4 have pkt, frequency bands 2,5,6 idle
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Random Access Protocols

When node has packet to send




two or more transmitting nodes ➜ “collision”,
random access MAC protocol specifies:



transmit at full channel data rate R.
no a priori coordination among nodes
how to detect collisions
how to recover from collisions (e.g., via delayed
retransmissions)
Examples of random access MAC protocols:



slotted ALOHA
ALOHA
CSMA, CSMA/CD, CSMA/CA
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Slotted ALOHA
Assumptions
 all frames same size
 time is divided into equal
size slots, time to transmit 1
frame
 nodes start to transmit
frames only at beginning of
slots
 nodes are synchronized
 if 2 or more nodes transmit
in slot, all nodes detect
collision
Operation
 when node obtains fresh
frame, it transmits in next
slot
 no collision, node can
send new frame in next
slot
 if collision, node
retransmits frame in each
subsequent slot with
prob. p until success
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Slotted ALOHA
Pros
 single active node can
continuously transmit
at full rate of channel
 highly decentralized:
only slots in nodes
need to be in sync
 simple
Cons
 collisions, wasting slots
 idle slots
 nodes may be able to
detect collision in less
than time to transmit
packet
 clock synchronization
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Slotted Aloha efficiency
Efficiency is the long-run
fraction of successful slots
when there are many nodes,
each with many frames to send


Suppose N nodes with many
frames to send, each
transmits in slot with
probability p
prob that node 1 has
success in a slot
= p(1p)N-1

prob that any node has a
success = Np(1-p)N-1
For max efficiency
with N nodes, find p*
that maximizes
Np(1-p)N-1
For many nodes, take
limit of Np*(1-p*)N-1 as
N goes to infinity,
gives 1/e = .37


At best: channel
used for useful
transmissions 37%
of time!
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Pure (unslotted) ALOHA


unslotted Aloha: simpler, no synchronization
when frame first arrives


transmit immediately
collision probability increases:

frame sent at t0 collides with other frames sent in [t0-1,t0+1]
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Pure Aloha efficiency
P(success by given node) = P(node transmits) .
P(no other node transmits in [p0-1,p0] .
P(no other node transmits in [p0-1,p0]
= p . (1-p)N-1 . (1-p)N-1
= p . (1-p)2(N-1)
… choosing optimum p and then letting n -> infty ...
Even worse !
= 1/(2e) = .18
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CSMA (Carrier Sense Multiple Access)
CSMA: listen before transmit:
If channel sensed idle: transmit entire frame
 If channel sensed busy, defer transmission

Human analogy: don’t interrupt others!
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CSMA collisions
spatial layout of nodes
collisions can still occur:
propagation delay means
two nodes may not hear
each other’s transmission
collision:
entire packet transmission
time wasted
note:
role of distance & propagation
delay in determining collision
probability
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CSMA/CD (Collision Detection)
CSMA/CD: carrier sensing, deferral as in CSMA



collision detection:



collisions detected within short time
colliding transmissions aborted, reducing channel
wastage
easy in wired LANs: measure signal strengths,
compare transmitted, received signals
difficult in wireless LANs: receiver shut off while
transmitting
human analogy: the polite conversationalist
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CSMA/CD collision detection
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“Taking Turns” MAC protocols
channel partitioning MAC protocols:
 share channel efficiently and fairly at high load
 inefficient at low load: delay in channel access, 1/N
bandwidth allocated even if only 1 active node!
Random access MAC protocols
 efficient at low load: single node can fully utilize channel
 high load: collision overhead
“taking turns” protocols
look for best of both worlds!
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“Taking Turns” MAC protocols
Polling:
 master node “invites”
slave nodes to transmit
in turn
 concerns:



polling overhead
latency
single point of failure
(master)
Token passing:
 control token passed from one
node to next sequentially.
 token message
 concerns:



token overhead
latency
single point of failure (token)
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Summary of MAC protocols

What do you do with a shared media?

Channel Partitioning, by time, frequency or code


Random partitioning (dynamic),





Time Division, Frequency Division
ALOHA, S-ALOHA, CSMA, CSMA/CD
carrier sensing: easy in some technologies (wire), hard
in others (wireless)
CSMA/CD used in Ethernet
CSMA/CA used in 802.11
Taking Turns

polling from a central site, token passing
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LAN technologies
Data link layer so far:

services, error detection/correction, multiple
access
Next: LAN technologies




addressing
Ethernet
hubs, switches
PPP
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Link Layer





5.1 Introduction and
services
5.2 Error detection and
correction
5.3Multiple access
protocols
5.4 Link-Layer
Addressing
5.5 Ethernet



5.6 Hubs and switches
5.7 PPP
5.8 Link Virtualization:
ATM
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MAC Addresses and ARP

32-bit IP address:



network-layer address
used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet)
address:


used to get datagram from one interface to
another physically-connected interface (same
network)
48 bit MAC address (for most LANs)
burned in the adapter ROM
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LAN Addresses and ARP
Each adapter on LAN has unique LAN address
1A-2F-BB-76-09-AD
71-65-F7-2B-08-53
LAN
(wired or
wireless)
Broadcast address =
FF-FF-FF-FF-FF-FF
= adapter
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
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LAN Address (more)




MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space
(to assure uniqueness)
Analogy:
(a) MAC address: like Social Security Number
(b) IP address: like postal address
MAC flat address ➜ portability


can move LAN card from one LAN to another
IP hierarchical address NOT portable

depends on IP subnet to which node is attached
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ARP: Address Resolution Protocol
Question: how to determine
MAC address of B
knowing B’s IP address?
237.196.7.78


1A-2F-BB-76-09-AD
237.196.7.23
237.196.7.14
LAN
71-65-F7-2B-08-53
237.196.7.88
58-23-D7-FA-20-B0
Each IP node (Host,
Router) on LAN has
ARP table
ARP Table: IP/MAC
address mappings for
some LAN nodes
< IP address; MAC address;
TTL>

0C-C4-11-6F-E3-98
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TTL (Time To Live): time
after which address
mapping will be forgotten
(typically 20 min)
40
ARP protocol: Same LAN (network)



A wants to send datagram to
B, and B’s MAC address not
in A’s ARP table.
A broadcasts ARP query
packet, containing B's IP
address
 Dest MAC address = FFFF-FF-FF-FF-FF
 all machines on LAN
receive ARP query
B receives ARP packet,
replies to A with its (B's)
MAC address

frame sent to A’s MAC
address (unicast)


A caches (saves) IP-toMAC address pair in its
ARP table until information
becomes old (times out)
 soft state: information
that times out (goes
away) unless refreshed
ARP is “plug-and-play”:

nodes create their ARP
tables without
intervention from net
administrator
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Routing to another LAN
walkthrough: send datagram from A to B via R
assume A know’s B IP address
A
R

Two ARP tables in router R, one for each IP
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network (LAN)
B
42








A creates datagram with source A, destination B
A uses ARP to get R’s MAC address for 111.111.111.110
A creates link-layer frame with R's MAC address as dest, frame
contains A-to-B IP datagram
A’s adapter sends frame
R’s adapter receives frame
R removes IP datagram from Ethernet frame, sees its destined
to B
R uses ARP to get B’s MAC address
R creates frame containing A-to-B IP datagram sends to B
A
R
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B
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Link Layer





5.1 Introduction and
services
5.2 Error detection and
correction
5.3Multiple access
protocols
5.4 Link-Layer
Addressing
5.5 Ethernet



5.6 Hubs and switches
5.7 PPP
5.8 Link Virtualization:
ATM
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Ethernet
“dominant” wired LAN technology:
 cheap $20 for 100Mbs!
 first widely used LAN technology
 Simpler, cheaper than token LANs and ATM
 Kept up with speed race: 10 Mbps – 10 Gbps
Metcalfe’s Ethernet
sketch
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Star topology



Bus topology popular through mid 90s
Now star topology prevails
Connection choices: hub or switch (more later)
hub or
switch
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Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or
other network layer protocol packet) in Ethernet
frame
Preamble:
 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011
 used to synchronize receiver, sender clock rates
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Ethernet Frame Structure (more)

Addresses: 6 bytes




if adapter receives frame with matching destination
address, or with broadcast address (eg ARP packet), it
passes data in frame to net-layer protocol
otherwise, adapter discards frame
Type: indicates the higher layer protocol (mostly IP
but others may be supported such as Novell IPX
and AppleTalk)
CRC: checked at receiver, if error is detected, the
frame is simply dropped
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Unreliable, connectionless service


Connectionless: No handshaking between sending
and receiving adapter.
Unreliable: receiving adapter doesn’t send acks or
nacks to sending adapter



stream of datagrams passed to network layer can have
gaps
gaps will be filled if app is using TCP
otherwise, app will see the gaps
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Ethernet uses CSMA/CD



No slots
adapter doesn’t transmit if
it senses that some other
adapter is transmitting,
that is, carrier sense
transmitting adapter aborts
when it senses that
another adapter is
transmitting, that is,
collision detection

Before attempting a
retransmission,
adapter waits a
random time, that is,
random access
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Ethernet CSMA/CD algorithm
1. Adaptor receives datagram
4. If adapter detects another
from net layer & creates frame
transmission while
2. If adapter senses channel idle,
transmitting, aborts and
it starts to transmit frame. If it
sends jam signal
senses channel busy, waits
5. After aborting, adapter
until channel idle and then
transmits
enters exponential
3. If adapter transmits entire
backoff: after the mth
frame without detecting
collision, adapter chooses
another transmission, the
a K at random from
adapter is done with frame !
m
{0,1,2,…,2 -1}. Adapter
waits K·512 bit times and
returns to Step 2
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Ethernet’s CSMA/CD (more)
Jam Signal: make sure all
other transmitters are
aware of collision; 48 bits
Bit time: .1 microsec for 10
Mbps Ethernet ;
for K=1023, wait time is
about 50 msec
See/interact with Java
applet on AWL Web site:
highly recommended !
Exponential Backoff:
 Goal: adapt
retransmission attempts to
estimated current load




heavy load: random wait
will be longer
first collision: choose K
from {0,1}; delay is K· 512
bit transmission times
after second collision:
choose K from {0,1,2,3}…
after ten collisions,
choose K from
{0,1,2,3,4,…,1023}
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CSMA/CD efficiency


Tprop = max prop between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency 



1
1  5t prop / ttrans
Efficiency goes to 1 as tprop goes to 0
Goes to 1 as ttrans goes to infinity
Much better than ALOHA, but still decentralized,
simple, and cheap
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10BaseT and 100BaseT



10/100 Mbps rate; latter called “fast ethernet”
T stands for Twisted Pair
Nodes connect to a hub: “star topology”; 100 m
max distance between nodes and hub
twisted pair
hub
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Hubs
Hubs are essentially physical-layer repeaters:
 bits coming from one link go out all other links
 at the same rate
 no frame buffering
 no CSMA/CD at hub: adapters detect collisions
 provides net management functionality
twisted pair
hub
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Manchester encoding




Used in 10BaseT
Each bit has a transition
Allows clocks in sending and receiving nodes to synchronize
to each other
 no need for a centralized, global clock among nodes!
Hey, this is physical-layer stuff!
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Gbit Ethernet






uses standard Ethernet frame format
allows for point-to-point links and shared broadcast
channels
in shared mode, CSMA/CD is used; short distances
between nodes required for efficiency
uses hubs, called here “Buffered Distributors”
Full-Duplex at 1 Gbps for point-to-point links
10 Gbps now !
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57
Link Layer





5.1 Introduction and
services
5.2 Error detection and
correction
5.3Multiple access
protocols
5.4 Link-Layer
Addressing
5.5 Ethernet



5.6 Interconnections:
Hubs and switches
5.7 PPP
5.8 Link Virtualization:
ATM
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Interconnecting with hubs




Backbone hub interconnects LAN segments
Extends max distance between nodes
But individual segment collision domains become one
large collision domain
Can’t interconnect 10BaseT & 100BaseT
hub
hub
hub
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hub
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Switch



Link layer device
 stores and forwards Ethernet frames
 examines frame header and selectively
forwards frame based on MAC dest address
 when frame is to be forwarded on segment,
uses CSMA/CD to access segment
transparent
 hosts are unaware of presence of switches
plug-and-play, self-learning
 switches do not need to be configured
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Forwarding
switch
1
2
hub
3
hub
hub
• How do determine onto which LAN segment to
forward frame?
• Looks like a routing problem...
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Self learning



A switch has a switch table
entry in switch table:
 (MAC Address, Interface, Time Stamp)
 stale entries in table dropped (TTL can be 60 min)
switch learns which hosts can be reached through
which interfaces
 when frame received, switch “learns” location of
sender: incoming LAN segment
 records sender/location pair in switch table
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Filtering/Forwarding
When switch receives a frame:
index switch table using MAC dest address
if entry found for destination
then{
if dest on segment from which frame arrived
then drop the frame
else forward the frame on interface indicated
}
forward on all but the interface
else flood
on which the frame arrived
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Switch example
Suppose C sends frame to D
1
B

2
hub
hub
1
1
2
3
I
D
E
F
G
H
Switch receives frame from from C



C
A
B
E
G
3
hub
A
address interface
switch
notes in bridge table that C is on interface 1
because D is not in table, switch forwards frame into
interfaces 2 and 3
frame received by D
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Switch example
Suppose D replies back with frame to C.
address interface
switch
B

I
D
E
F
G
1
1
2
3
1
H
Switch receives frame from from D



C
hub
hub
hub
A
A
B
E
G
C
notes in bridge table that D is on interface 2
because C is in table, switch forwards frame only to interface 1
frame received by C
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Switch: traffic isolation


switch installation breaks subnet into LAN
segments
switch filters packets:
 same-LAN-segment frames not usually
forwarded onto other LAN segments
 segments become separate collision domains
switch
collision
domain
hub
collision domain
hub
University of Technology
collisionSharif
domain
hub
66
Switches: dedicated access



Switch with many
interfaces
Hosts have direct
connection to switch
No collisions; full duplex
A
C’
Switching: A-to-A’ and B-to-B’
simultaneously, no
collisions
Sharif University of Technology
B
switch
C
B’
A’
67
More on Switches


cut-through switching: frame forwarded from
input to output port without first collecting
entire frame
 slight reduction in latency
combinations of shared/dedicated,
10/100/1000 Mbps interfaces
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Institutional network
to external
network
mail server
web server
router
switch
IP subnet
hub
hub
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hub
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Switches vs. Routers

both store-and-forward devices




routers: network layer devices (examine network layer
headers)
switches are link layer devices
routers maintain routing tables, implement routing
algorithms
switches maintain switch tables, implement
filtering, learning algorithms
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Summary comparison
hubs
routers
switches
traffic
isolation
no
yes
yes
plug & play
yes
no
yes
optimal
routing
cut
through
no
yes
no
yes
no
yes
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Link Layer





5.1 Introduction and
services
5.2 Error detection and
correction
5.3Multiple access
protocols
5.4 Link-Layer
Addressing
5.5 Ethernet



5.6 Hubs and switches
5.7 PPP
5.8 Link Virtualization:
ATM
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Point to Point Data Link Control


one sender, one receiver, one link: easier than
broadcast link:
 no Media Access Control
 no need for explicit MAC addressing
 e.g., dialup link, ISDN line
popular point-to-point DLC protocols:
 PPP (point-to-point protocol)
 HDLC: High level data link control (Data link
used to be considered “high layer” in protocol
stack!
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PPP Design Requirements [RFC 1557]





packet framing: encapsulation of network-layer
datagram in data link frame
 carry network layer data of any network layer
protocol (not just IP) at same time
 ability to demultiplex upwards
bit transparency: must carry any bit pattern in the
data field
error detection (no correction)
connection liveness: detect, signal link failure to
network layer
network layer address negotiation: endpoint can
learn/configure each other’s network address
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PPP non-requirements




no error correction/recovery
no flow control
out of order delivery OK
no need to support multipoint links (e.g., polling)
Error recovery, flow control, data re-ordering
all relegated to higher layers!
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PPP Data Frame




Flag: delimiter (framing)
Address: does nothing (only one option)
Control: does nothing; in the future possible multiple
control fields
Protocol: upper layer protocol to which frame
delivered (eg, PPP-LCP, IP, IPCP, etc)
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PPP Data Frame


info: upper layer data being carried
check: cyclic redundancy check for error detection
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Byte Stuffing

“data transparency” requirement: data field must
be allowed to include flag pattern <01111110>
 Q: is received <01111110> data or flag?

Sender: adds (“stuffs”) extra < 01111110> byte
after each < 01111110> data byte
Receiver:
 two 01111110 bytes in a row: discard first byte,
continue data reception
 single 01111110: flag byte

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Byte Stuffing
flag byte
pattern
in data
to send
flag byte pattern plus
stuffed byte in
transmitted data
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PPP Data Control Protocol
Before exchanging networklayer data, data link peers
must
 configure PPP link (max.
frame length,
authentication)
 learn/configure network
layer information
 for IP: carry IP Control
Protocol (IPCP) msgs
(protocol field: 8021) to
configure/learn IP
address
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Link Layer





5.1 Introduction and
services
5.2 Error detection and
correction
5.3Multiple access
protocols
5.4 Link-Layer
Addressing
5.5 Ethernet



5.6 Hubs and switches
5.7 PPP
5.8 Link Virtualization:
ATM and MPLS
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Virtualization of networks
Virtualization of resources: a powerful abstraction in
systems engineering:
 computing examples: virtual memory, virtual devices
 Virtual machines: e.g., java
 IBM VM os from 1960’s/70’s
 layering of abstractions: don’t sweat the details of
the lower layer, only deal with lower layers abstractly
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The Internet: virtualizing networks
1974: multiple unconnected
nets
ARPAnet
 data-over-cable networks
 packet satellite network (Aloha)
 packet radio network

… differing in:
addressing conventions
 packet formats
 error recovery
 routing

ARPAnet
"A Protocol for Packet Network Intercommunication",
V. Cerf, R. Kahn, IEEE Transactions on Communications,
Sharif University of Technology
May, 1974, pp. 637-648.
satellite net
83
The Internet: virtualizing networks
Internetwork layer (IP):
 addressing: internetwork
appears as a single, uniform
entity, despite underlying local
network heterogeneity
 network of networks
Gateway:
 “embed internetwork packets in
local packet format or extract
them”
 route (at internetwork level) to
next gateway
gateway
satellite net
ARPAnet
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Cerf & Kahn’s Internetwork Architecture
What is virtualized?



two layers of addressing: internetwork and local
network
new layer (IP) makes everything homogeneous
at internetwork layer
underlying local network technology
 cable
 satellite
 56K telephone modem
 today: ATM, MPLS
… “invisible” at internetwork layer. Looks like a
link layer technology to IP!
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85
ATM and MPLS

ATM, MPLS separate networks in their own
right


viewed by Internet as logical link connecting
IP routers


different service models, addressing, routing
from Internet
just like dialup link is really part of separate
network (telephone network)
ATM, MPSL: of technical interest in their
own right
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Asynchronous Transfer Mode: ATM


1990’s/00 standard for high-speed (155Mbps to
622 Mbps and higher) Broadband Integrated
Service Digital Network architecture
Goal: integrated, end-end transport of carry voice,
video, data
 meeting timing/QoS requirements of voice,
video (versus Internet best-effort model)
 “next generation” telephony: technical roots in
telephone world
 packet-switching (fixed length packets, called
“cells”) using virtual circuits
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ATM architecture



adaptation layer: only at edge of ATM network
 data segmentation/reassembly
 roughly analagous to Internet transport layer
ATM layer: “network” layer
 cell switching, routing
physical layer
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ATM: network or link layer?
Vision: end-to-end
transport: “ATM from
desktop to desktop”
 ATM is a network
technology
Reality: used to connect
IP backbone routers
 “IP over ATM”
 ATM as switched
link layer,
connecting IP
routers
IP
network
ATM
network
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ATM Adaptation Layer (AAL)



ATM Adaptation Layer (AAL): “adapts” upper
layers (IP or native ATM applications) to ATM layer
below
AAL present only in end systems, not in switches
AAL layer segment (header/trailer fields, data)
fragmented across multiple ATM cells
 analogy: TCP segment in many IP packets
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ATM Adaptation Layer (AAL) [more]
Different versions of AAL layers, depending on ATM
service class:



AAL1: for CBR (Constant Bit Rate) services, e.g. circuit emulation
AAL2: for VBR (Variable Bit Rate) services, e.g., MPEG video
AAL5: for data (eg, IP datagrams)
User data
AAL PDU
ATM cell
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ATM Layer
Service: transport cells across ATM network
 analogous to IP network layer
 very different services than IP network layer
Network
Architecture
Internet
Service
Model
Guarantees ?
Congestion
Bandwidth Loss Order Timing feedback
best effort none
ATM
CBR
ATM
VBR
ATM
ABR
ATM
UBR
constant
rate
guaranteed
rate
guaranteed
minimum
none
no
no
no
yes
yes
yes
yes
yes
yes
no
yes
no
no (inferred
via loss)
no
congestion
no
congestion
yes
no
yes
no
no
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ATM Layer: Virtual Circuits

VC transport: cells carried on VC from source to dest






call setup, teardown for each call before data can flow
each packet carries VC identifier (not destination ID)
every switch on source-dest path maintain “state” for each
passing connection
link,switch resources (bandwidth, buffers) may be allocated to
VC: to get circuit-like perf.
Permanent VCs (PVCs)
 long lasting connections
 typically: “permanent” route between to IP routers
Switched VCs (SVC):
 dynamically set up on per-call basis
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ATM VCs


Advantages of ATM VC approach:
 QoS performance guarantee for connection
mapped to VC (bandwidth, delay, delay jitter)
Drawbacks of ATM VC approach:
 Inefficient support of datagram traffic
 one PVC between each source/dest pair) does
not scale (N*2 connections needed)
 SVC introduces call setup latency, processing
overhead for short lived connections
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ATM Layer: ATM cell


5-byte ATM cell header
48-byte payload
 Why?: small payload -> short cell-creation
delay for digitized voice
 halfway between 32 and 64 (compromise!)
Cell header
Cell format
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ATM cell header




VCI: virtual channel ID
 will change from link to link thru net
PT: Payload type (e.g. RM cell versus data cell)
CLP: Cell Loss Priority bit
 CLP = 1 implies low priority cell, can be
discarded if congestion
HEC: Header Error Checksum
 cyclic redundancy check
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ATM Physical Layer (more)
Two pieces (sublayers) of physical layer:
 Transmission Convergence Sublayer (TCS): adapts
ATM layer above to PMD sublayer below
 Physical Medium Dependent: depends on physical
medium being used
TCS Functions:
 Header checksum generation: 8 bits CRC
 Cell delineation
 With “unstructured” PMD sublayer, transmission
of idle cells when no data cells to send
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ATM Physical Layer
Physical Medium Dependent (PMD) sublayer

SONET/SDH: transmission frame structure (like a
container carrying bits);
 bit synchronization;
 bandwidth partitions (TDM);
 several speeds: OC3 = 155.52 Mbps; OC12 = 622.08
Mbps; OC48 = 2.45 Gbps, OC192 = 9.6 Gbps


TI/T3: transmission frame structure (old telephone
hierarchy): 1.5 Mbps/ 45 Mbps
unstructured: just cells (busy/idle)
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IP-Over-ATM
Classic IP only
 3 “networks” (e.g.,
LAN segments)
 MAC (802.3) and
IP addresses
IP over ATM
 replace “network”
(e.g., LAN segment)
with ATM network
 ATM addresses, IP
addresses
ATM
network
Ethernet
LANs
Ethernet
LANs
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IP-Over-ATM
app
transport
IP
Eth
phy
IP
AAL
Eth
ATM
phy phy
ATM
phy
ATM
phy
Sharif University of Technology
app
transport
IP
AAL
ATM
phy
100
Datagram Journey in IP-over-ATM Network

at Source Host:





IP layer maps between IP, ATM dest address (using ARP)
passes datagram to AAL5
AAL5 encapsulates data, segments cells, passes to ATM layer
ATM network: moves cell along VC to destination
at Destination Host:
 AAL5 reassembles cells into original datagram
 if CRC OK, datagram is passed to IP
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IP-Over-ATM
Issues:
 IP datagrams into
ATM AAL5 PDUs
 from IP addresses
to ATM addresses
 just like IP
addresses to
802.3 MAC
addresses!
ATM
network
Ethernet
LANs
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Multiprotocol label switching (MPLS)

initial goal: speed up IP forwarding by using fixed
length label (instead of IP address) to do forwarding


borrowing ideas from Virtual Circuit (VC) approach
but IP datagram still keeps IP address!
PPP or Ethernet
header
MPLS header
label
20
IP header
remainder of link-layer frame
Exp S TTL
3
1
5
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MPLS capable routers


a.k.a. label-switched router
forwards packets to outgoing interface based only on
label value (don’t inspect IP address)


signaling protocol needed to set up forwarding




MPLS forwarding table distinct from IP forwarding tables
RSVP-TE
forwarding possible along paths that IP alone would not
allow (e.g., source-specific routing) !!
use MPLS for traffic engineering
must co-exist with IP-only routers
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MPLS forwarding tables
in
label
out
label dest
10
12
8
out
interface
A
D
A
0
0
1
in
label
out
label dest
out
interface
10
6
A
1
12
9
D
0
R6
0
0
D
1
1
R3
R4
R5
0
0
R2
in
label
8
out
label dest
6
A
out
interface
in
label
6
0
Sharif University of Technology
outR1
label dest
-
A
A
out
interface
0
105
Chapter 5: Summary

principles behind data link layer services:




error detection, correction
sharing a broadcast channel: multiple access
link layer addressing
instantiation and implementation of various link
layer technologies
 Ethernet
 switched LANS
 PPP
 virtualized networks as a link layer: ATM, MPLS
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